Process industries today are faced with increasingly stringent environmental regulations with regard to fluid leakage and containment of process fluids. At the same time, demands for increased productivity require higher reliability equipment to minimize down time. In an effort to meet both of these challenges, many in the process industry have moved from a conventional centrifugal motor/pump configuration to sealless motor/pumps. This movement is precipitated in part due to the problem of wear of the mechanical seals required by the conventional centrifugal motor/pumps. This seal wear may result in increased leakage rates of the process fluid or massive failure.
A canned motor/pump assures total fluid containment by eliminating any moving part extending through the primary containment or can, therefore eliminating the seal wear/leakage problem of the conventional centrifugal motor/pumps. The canned motor/pump, as its name implies, comprises a sealed housing (primary containment) or can into which is placed the rotor portion of the drive motor. The rotor shaft is coupled to and drives the pump impeller at the pump end of the machine. The can is open to the pump end of the machine, and hence process fluid is allowed to circulate through the can and around the rotor. The rotor is supported within this can by its shaft on radial journal and axial thrust bearings which are lubricated by the circulating process fluid within the can.
While sealless pumps solve the seal wear/leakage problems of conventional motor/pumps, they are susceptible to catastrophic failure. This catastrophic failure is most often the result of excessive bearing wear, dry run, or high vibration. As stated above, a typical sealless or canned motor pump has its motor rotor supported on radial journal and axial thrust bearings lubricated with process fluid. These bearings are made of a relatively soft carbon or wear resistant ceramic material. Lubrication is marginal for most operating conditions due to the nature of the process fluid and flow paths within the can. Two key factors that impact carbon bearings wear are abrasives in the fluid and intermittent dry operation. If the bearings wear beyond a prescribed limit, contact can occur between the rotor and stator causing severe damage to the pump. On the other hand, motors with ceramics bearings can fail due to dry operation which results in thermal shock and sudden failure of the brittle bearing material. Excessive shaft vibration due to rotor imbalance or bearing instability can also result in accelerated bearing wear. Incorrect direction of rotation results in reduced pump performance and potential bearing failure due to lack of cooling or lubricating fluid flow.
Since the bearings in a canned motor pump are contained within the sealed can, visual inspection for excessive wear is impossible without shutting down the pump and dismantling the machine. An external mechanical bearing wear monitoring device, such as shown in U.S. Pat. No. 3,991,701, entitled Bearing Wear Detecting Device for Canned Motor Driven Pumps, issued to Sato, has been used to warn of impending failure in a canned motor pump. This device, however, detects only when the bearings have worn approximately two-thirds of the allowable wear. The wear detector of the Sato '701 patent does not indicate progressive wear or, which bearing is being worn. Indication of wear of the bearing on the shaft end opposite the bearing wear monitor is not always reliable. It also cannot indicate dry run operation excessive vibration or reverse rotation which are also causes of bearing failure. Additionally, once this type of device activates, it must be replaced. These features are no longer acceptable to customers in the industry.
In recognition of the problems and dissatisfaction associated with such devices, electronic bearing monitoring devices which allow the detection and indication of progressive bearing wear began to be utilized by canned pump manufacturers. One such device is shown in British Patent No. 1,480,848 issued to Considine, for Improvements in or Relating to Bearing Wear Detection Devices, on Oct. 8, 1973. The Considine '848 device utilizes a single electronic coil positioned completely around a bobbin which is drivably coupled to the motor shaft of a canned motor pump. The bobbin is mounted within an outer housing which is mounted on the end of the housing for the canned motor pump. The bobbin contains permanent magnets, and any eccentricity in rotation will induce an emf in the coil. This emf signal is passed through an aperture in the can, which is filled with encapsulating resin after final assembly, to a detection circuit.
While this device allows for detection of progressive bearing wear, it does so only for bearing wear in a radial direction on one end. Disadvantageously, axial bearing wear is not detected by the eccentric rotation of the bobbin. Additionally, the addition of an outer housing on the end of the canned motor pump is disadvantageous. The end where the bobbin is to be mounted is normally closed as part of the containment can for the motor pump. The addition of the outer housing requires the addition of a gasket to seal the junction between the containment can and the outer housing. This introduces one more point of possible leakage for the sealless motor pump. Additionally, in many processing plant operations, the installation envelope of a canned motor pump is limited, and the addition of the bobbin's outer housing increases the overall length of the machine.
One manufacturer of canned motor pumps, Teikoku U.S.A., Inc., also utilizes electronic monitoring devices known as the Teikoku Rotary Guardian (TRG). One aspect of the TRG Monitor is disclosed by U.S. Pat. No. 4,211,973, for an Apparatus for Detecting Faults to be Occurred or Initially Existing in a Running Electric Rotary Machine, issued to Sato et al., on Jul. 8, 1980. This electronic monitoring device detects eccentric rotation of the rotor due to radial bearing wear. It comprises two coils specially located within the stator slots of the main stator and coupled in series. Energization of the main stator windings provides the magnetic flux which is utilized by the detector windings. The coils are coupled in series to cancel the fundamental component of the stator energization and to allow detection of eccentric rotation. Unfortunately, this detection circuitry cannot detect bearing wear in a thrust or axial direction. Additionally, it cannot be used with a conventional canned motor pump configuration, but requires that the fundamental structure of the motor must be adapted to allow cooperation with the detector coils. This greatly limits its ability to be added as a retrofit to existing canned motor pumps already in operation in the process industry.
The other patent referenced by advertisements for TRG is U.S. Pat. No. 4,334,189, for Operation Supervisory Apparatus, issued to Sato, on Jun. 8, 1982. This patent discloses a similar bearing wear detector utilizing two coils interposed in the stator slots as disclosed in the Sato et al. '973 patent. In addition to the bearing wear monitor, this reference additionally adds a direction of rotation monitor of conventional design, and a combining circuit to output the larger of the two monitor's outputs. As with the '973 reference, however, the bearing monitor only monitors wear in a radial direction. No monitoring is provided for bearing wear in an axial or thrust direction. Additionally, the ability to easily retrofit existing canned motor pumps to incorporate these detectors is greatly reduced due to the location of the monitor coils (within the stator slots), and due to the requirement of a specific rotor/stator structure to allow cooperation of the detector coils.
In recognition of the deficiencies of the foregoing references in failing to provide indication of bearing wear in both an axial and a radial direction, bearing monitors which were capable of detecting both directions of wear were developed. One such bearing wear monitor is disclosed in U.S. Pat. No. 5,189,763, issued to Konishi, for an Apparatus for Monitoring the Axial and Radial Wear on a Bearing of a Rotary Shaft, on Mar. 30, 1993. This monitoring device utilizes a single contactless distance sensor and an axial groove or ridge and a helicial groove or ridge. Alternatively, Konishi '763 discloses that other devices such as a light source and a photo sensor may be used in a similar manner. The grooves or ridges are formed on a rotor shaft end nut, and the distance sensor is placed in close proximity to the end nut, within the containment can. While this device provides both axial and radial bearing wear indication, it still requires that the containment can be penetrated by a probe containing the distance sensor and electrical leads. While this probe is not in motion, sealing and potential leaks are still a problem.
It is a primary objective of the instant invention, therefore, to solve the aforementioned problems by providing accurate and continuous monitoring of progressive radial and axial rotor position and progressive bearing wear when the shaft is moving eccentrically or concentrically, measurement of excessive rotor vibration, and detection of incorrect direction of rotation without penetrating the containment can of a canned motor pump, and without requiring a change in the fundamental structure of the rotor and stator of a canned motor pump. It is also an objective of the instant invention to provide data on demand for the purpose of diagnostic or trend analysis to aid users in the process industry in planning maintenance activity.
It is a further objective of the instant invention to sense and display the progressive axial bearing wear in both directions along the axis of rotation. It is an added objective to indicate in which of these two directions axial wear is actually occurring. It is a still further objective of the instant invention to sense and display progressive radial bearing wear for each end of the rotor shaft. It is a further objective of the instant invention to provide a non-invasive means to sense both progressive bearing wear in both directions of axial wear and radial wear on both ends of the rotor shaft simultaneously. It is additionally an objective of the instant invention to provide a means for sensing this type of bearing wear which is easily retrofitable on existing motor pumps currently being utilized in the process industry. It is also an objective of the instant invention to provide an effective means of sensing this bearing wear in an extremely electromagnetically noisy environment. It is also an objective of the instant invention to provide a sensor means that is low cost to manufacture.